WASHINGTON — The Atlanta Braves have run far ahead of their NL East foes this season.

Now they’re stumbling to the finish.

The Washington Nationals broke open a one-run game with a three-run eighth off reliever Jordan Walden and defeated the Braves 4-0 Tuesday night for a sweep of the day-night doubleheader.

In the makeup of a series opener postponed by a shooting rampage a day earlier at the nearby Navy Yard, the Nationals scored three times off Atlanta closer Craig Kimbrel in the ninth for a 6-5 win in the first game.

“We need to start playing a little better baseball and start swinging the bats,” said manager Fredi Gonzalez, whose hitters collected just five singles in the nightcap. “I like where we are, our position, but there’s still baseball games to be played, and we’ve just got to get back on a roll again.”

Atlanta has lost nine of 13 and the magic number remained at four for clinching the NL East for the first time since the Braves’ streak of 14 consecutive division titles from 1991-2005, not including the strike-shortened ’94 season. A sweep of the day-night doubleheader would have sealed the division, now Atlanta’s lead over Washington is eight games, the smallest it’s been since July 25.

Washington’s Tanner Roark (7-0) allowed just two hits in seven innings, struck out six and retired the final 13 batters he faced. Roark has yet to allow a run in 13 innings against the Braves.

Veteran right-hander Freddy Garcia (1-2), making his second start for Atlanta, allowed a run on seven hits.

“He threw the ball real well,” catcher Gerald Laird said. “I remember facing him a lot with Detroit, when he was with the White Sox, and he’s one of those guys who keeps you off balance and doesn’t make mistakes in the middle of the plate.”

After allowing Steve Lombardozzi’s RBI-single in the second, he worked out of trouble in the third and fourth, and retired the last eight men he faced.

Washington, which began the game 4½ games behind Cincinnati for the NL’s second wild-card berth, added three runs, including a Ryan Zimmerman homer, off Walden in the eighth.

“You want to keep it a one-run ballgame and you bring the guy in you want in that situation and it just didn’t happen (Tuesday) for us,” Gonzalez said.

The Braves wasted a two-run, ninth-inning lead in an opening 6-5 loss.

Atlanta trailed 3-0 in the first but took a 4-3 lead in the eighth on Evan Gattis’ two-run homer off Tyler Clippard on an 0-2 pitch, which stopped an 0-for-18 skid. The Braves added a run in the ninth when Elliot Johnson singled, advanced on a throwing error by left fielder Bryce Harper, stole third and came around as Ian Desmond allowed Freddie Freeman’s four-hop grounder to shortstop to bounce off the heel of his glove for his 18th error this season.

In the bottom half, Adam LaRoche walked on a full count, Wilson Ramos reached on an infield single and Anthony Rendon walked on a 3-2 pitch. After Tracy’s grounder, Atlanta elected to pitch to Span with first base open, and he hit a three-hopper over the mound that rolled past Simmons into center field. Span was given an RBI, with the second run scoring on the error.

“With Craig, it can be bases loaded no outs and we still feel like we’re going to get out of it and he’s going to strike out the side,” starter Mike Minor said.

Simmons began the day with second-highest fielding percentage among NL shortstops.

“I just didn’t do my job. I didn’t catch the ball,” he said

Kimbrel (3-3), who leads the major leagues with 47 saves, had converted 37 in a row but blew a save chance for just the fourth time this season.

Ian Krol (2-1) faced two batters in the ninth, giving Washington its first win in seven home games against Atlanta this year.

Before the games, both teams stood in front of their dugouts, heads bowed, and a color guard was stationed behind home plate during a minute of silence to pay tribute to the people killed Monday and those affected by the shootings.

Span picked up a single in each game, extending his hitting streak to 28 games, passing Colorado’s Michael Cuddyer for the longest in the major leagues this season.

NOTES: With a sacrifice fly in the first game, Freddie Freeman became the first Atlanta player with 100 RBIs in a season since Chipper Jones and Jeff Francoeur in 2007. … Adm. James Winnefeld, the vice chairman of the Joint Chiefs of Staff, greeted players in the Nationals clubhouse before the game, handing out blue and gold Navy caps to wear during warmups.

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